do_copy() cannot run be asynchronously. Sort it out.

Pony ride on DistributionJob, where the distribution and distroseries
are *destinations*. do_copy()'s signature is:

def do_copy(sources, archive, series, pocket, include_binaries=False,
            allow_delayed_copies=True):

It runs quickly, so there's no need to check on progress, so it
doesn't matter much that this doesn't map well onto DistributionJob.

Important: calls to do_copy() must not be made concurrently for the
same target archive. For now ensure that the job runner runs all
do_copy() jobs serially.